Steel parts for machine structure use, for example, gears of automatic transmissions, sheaves of continuously variable transmissions, constant velocity joints, hubs, bearings and other power transmission parts require high contact fatigue strength.
In the past, in general, the above parts have been obtained by working materials such as JIS SCr420, SCM420, or other C: 0.2% or so case hardened steel into parts, then treating them by carburized quenching to form C: 0.8% or so martensite structure hardened layers on the surfaces of the parts and improve the contact fatigue strength.
However, carburized quenching is accompanied with austenite transformation at a 950° C. or so high temperature and is treated for a long period of 5 to 10 hours, in some cases, 10 hours or more, so larger heat treatment deformation due to coarsening of the crystal grains (distortion during quenching) is difficult to avoid.
For this reason, when a steel part requires high precision, after carburized quenching, the steel part has to be ground, honed, or otherwise finished.
In addition to this, in recent years, there has been a rising demand for reduction of the noise of automobile engines etc., so surface hardening methods with smaller heat distortion compared with carburized quenching such as induction hardening and soft nitriding have come into focus.
Induction hardening is a method of austenizing and quenching only the required part of the surface layer part of a steel material by heating for a short time and enables a surface hardened part with small distortion during quenching to be obtained with good precision. However, if trying to use just induction hardening to obtain a hardness equivalent to that of a carburized quenched material, 0.8% or more of C has to be added.
If the amount of C in the steel becomes 0.8% or more, the internal hardness, which is unnecessary for improvement of the contact fatigue strength, also rises and the machineability remarkably deteriorates, so it is not possible to just increase the amount of C in the steel. There are limits to improving the contact fatigue strength by just induction hardening.
Soft nitriding is a surface hardening method mainly causing nitrogen and carbon to simultaneously diffuse in and permeate the surface of a steel material to form a hardened layer in the 500 to 600° C. temperature range, which is less than the transformation point, and improve the wear resistance, seize resistance, fatigue resistance, etc.
The surface of the steel material is formed with nitrides by the diffused nitrogen. Usually, at the surfacemost layer of the steel material, a compound layer mainly comprised of Fe3N, Fe4N, or other Fe nitrides is formed. Inside, a nitrided layer with N diffused in it is formed.
Soft nitriding can be performed at a low temperature. Further, compared with carburization, the treatment time is a short one of 2 to 4 hours or so, therefore this is often used for the production of steel parts requiring low distortion. However, with just soft nitriding, the hardened layer depth is shallow, so this cannot be used for transmission gears etc. to which a high contact pressure is applied.
In recent years, as a technique making up for the defects of induction hardening and soft nitriding and giving better mechanical properties, in particular better contact fatigue strength, performing soft nitriding, then induction hardening has been experimented with (see PLT's 1 to 7).
For example, PLT 1 discloses the method of combining induction hardening and gas soft nitriding to make up for their respective defects and obtain excellent mechanical properties, in particular, high contact fatigue strength by improvement of the softening resistance.
However, with the method of PLT 1, the surface hardness is high, but the concentration of N in the nitrided layer is low, so the high temperature hardness is low, sufficient softening resistance cannot be obtained at the surface of the gear etc. becoming high in temperature during operation, and in the final analysis a high contact fatigue strength cannot be obtained.
PLT 2 also discloses the method of combining induction hardening and soft nitriding to produce parts for machine structure use excellent in mechanical strength. With the method of PLT 2, to enable the nitrides to form a solid solution, 900° C. to 1200° C. high temperature induction heating is necessary.
However, the amounts of addition of elements with a high affinity with N which promote the breakdown and dispersion of nitrides are insufficient, so high temperature heating is required. Therefore, the surface of the steel material is formed with an oxide layer to a remarkable extent and the mechanical properties end up greatly deteriorating.
Further, with the method of PLT 2, no consideration is given to the method of forming a thick compound layer, so good contact fatigue strength cannot be obtained under a high contact pressure.
PLT 3 discloses a method of production of a part for machine structure use excellent in mechanical strength characterized by treating steel comprised of, by wt %, C: 0.35 to 0.65%, Si: 0.03 to 1.50%, Mn: 0.3 to 1.0%, Cr: 0.1 to 3.0%, and a balance of Fe and impurities by soft nitriding under conditions giving a nitrided layer depth of 150 μm or more, then by induction hardening under conditions where the nitrided layer austenizes.
However, in the method of production of PLT 3, no consideration is given to raising the contact fatigue strength by the formation of a required thickness of a nitrided layer.
PLT 4 discloses a method of heat treatment of a machine part characterized by soft nitriding an iron-based material worked into the shape of a part so as to make nitrogen diffuse in and permeate the surface layer and form a compound layer, then induction hardening the part under conditions where the compound layer is consumed, the diffusion layer of the newly formed surface layer is denitrided, and a porous layer is formed at the surfacemost part.
However, in the heat treatment method of PLT 4, no consideration is given to raising the contact fatigue strength by the formation of a required thickness of a nitrided layer.
PLT 5 discloses a roller support shaft used for a cam follower device made of an iron-based alloy containing Cr, Mo, V, and W in a total of 1.0 to 20.0 wt % and C and N in a total of 0.5 to 1.2 wt % and having a balance of unavoidable impurities and Fe, nitrided at its surface, then induction quenched at the outer peripheral parts other than the two ends.
However, in the roller support shaft of PLT 5, no consideration is given to raising the contact fatigue strength by the formation of a required thickness of a nitrided layer.
PLT 6 also discloses a method of combining induction hardening and nitriding to obtain excellent mechanical properties. However, the nitriding in the method of PLT 6 is performed at a high temperature of 600° C. or more, so the compound layer is thin. Furthermore, the N concentration in it is low, so the amount of N diffusing due to decomposition at the time of induction hardening is also small.
In the end, with the nitriding of PLT 6, while a compound layer can be formed, formation of a thick, high N concentration nitrided layer is difficult, so even if combined with induction hardening, formation of a high softening resistance, good contact fatigue strength nitrided layer is not possible.
PLT 7 discloses steel for machine structure use excellent in strength, ductility, toughness, and wear resistance characterized by containing, by mass %, C: over 0.30%, 0.50% or less, Si: 1.0% or less, Mn: 1.5% or less, Mo: 0.3% to 0.5%, Ti: 0.1% or less, and B: 0.0005% to 0.01%, having a balance of Fe and unavoidable impurities, having at its surface a hardened layer of a thickness 50 μm or less and a Vicker's hardness of 750 or more, and having structures other than said hardened layer with an old austenite grain size of 10 μm or less, a martensite percentage of 90% or more, and a Vicker's hardness of 450 to less than 750.
However, the steel for machine structure use of PLT 7 does not form the required thickness of nitrided layer and raise the contact fatigue strength, so even if this can be applied to a metal belt of a continuously variable transmission, it is difficult to apply this to gears of automatic transmissions, sheaves of continuously variable transmissions, constant velocity joints, hubs, and other power transmission parts subjected to high contact pressures.
Whatever the case, steel for structural use for surface hardening able to be used for power transmission parts subjected to high contact pressures has not been provided up to now.